In order to make clear failure mechanism of steel pipe open sabo dam occurred at Nagiso in 2014, the miniture model experiments are carried out. Following suggestion reported by investigation committee, a dent deformation effect of steel pipe which reduces axial resistance of horizontal steel pipe is examined in addition to effects of pipe joint strength and preset sedimentary gravel at upstream side of the structure. Among these nine experimental cases in the dent deformation model, damage occurred only in the case which the joint part is 70 % strength to the base pipe and the initial sediment is 30 % of the dam height. A detail mechanism of failure and dent deformation effect i s examined through the observation of experimental results.
